After a well borehole has been drilled to a specified depth, a perforating shaped charge is used to form a jet perforation extending radially outwardly which punctures casing in the well, cement on the exterior and adjacent formations with the view of initiating fluid flow from the formation of interest. It is an important sequential step, which if misfired, creates a great deal of risk in the well completion procedures.
A typical procedure is to support an assembly on a wireline which incorporates one or more (typically several) jet perforating shaped charges. A detonator is included to trigger operation of the various shaped charges. In one approach, the shaped charges are supported on an open carrier which has the form of a lengthwise metallic strip or the like. The shaped charges are exposed to well fluids. A detonator supported on the carrier is also exposed to the well fluids and must operate with impunity to the surrounding environment. The detonator (sometimes called a blasting cap) starts detonation at one end of a detonating cord which extends the length of the apparatus. The resulting shock wave formed by the detonator travels along the cord and initiates the multiple attached shaped charges to perforate the well. The detonator is ordinarily constructed of a primary explosive material. The terms "primary" and "secondary" refer to the relative sensitivity of explosive materials. A typical primary explosive material is lead azide, and another is lead styphnate. Such primary explosive materials are normally extremely sensitive to any stimuli, including heat, sparks, friction, shock, and electrical current. To the measure that they are somewhat sensitive to various stimuli, a safety hazard is created in light of the fact that any stimuli may trigger premature detonation. This type of sensitivity associated with well known primary explosives shows them to be sensitive to premature or unintended shock, static electricity discharge, high ambient temperature normally associated with downhole conditions and other causes of detonation. For instance, electromagnetic radiation is a serious factor including RF (Radio Frequency) radiation at any wave length. Electrical static and mechanical agitation can also cause premature triggering. Detonation may occur at the wrong depth in the well and place the perforations at the wrong location. It may also occur near the well head, and possibly injure personnel near the well head.
The term "secondary explosive" refers to explosive materials which are not as sensitive as primary explosive materials. Typical examples of secondary explosives are RDX, HNS, PYX and others. In general terms, they are much more stable for handling and are relatively insensitive to detonation initiation. This lack of sensitivity makes them much safer to use. They are much safer to handle and are not as likely to explode prematurely. Secondary explosive detonators are much safer from inadvertent operation. In fact, they are so difficult to detonate that it requires special effort to provide proper detonation shock. In detonators made of primary explosives, the current for detonation is typically less than one ampere. This is so small as to run the risk of detonation with stray currents in the firing circuit. This also suggests that these detonators are sensitive to heat, impact, and unintended static discharge. With a secondary explosive, a significantly greater current flow (or other external stimuli) is required for detonation.
The present invention sets forth a means and method of detonating secondary explosives in a detonator which particularly protects against unintended stray currents, static electricity discharges, and the like. It also protects against RF detonation. It additionally protects against unintended shock detonation. The present apparatus contemplates the use of an electrically operated detonator which is provided with a sizable current over a long interval of time. While it is possible for a static discharge to ignite a secondary explosive, it is highly unlikely. Secondary explosives in the detonator inevitably require a much larger electric current for initiation. The present apparatus incorporates a system whereby a large AC current is applied through a wireline to a circuit which forms a proper detonation signal. The signal is delivered in the form of AC current flow which is stored on a charging capacitor through a voltage multiplying circuit. Only when the current forms an adequate charge is the capacitor able to form a discharge ideally through a gas discharge tube, or spark gap. This circuit cooperatively yields a charging sequence which forms an adequate charge, a charge having a voltage exceeding a required minimum and sustains the current on discharge for at least a specified interval. The circuit protects against stray or static discharges. Static which occurs in random fashion may create a momentary charge on the capacitor but that is reduced to zero by a bleed circuit incorporating a resistor connected to ground. As the circuit operates protectively, no preliminary inadvertent triggering event can occur whereby premature detonation occurs. Thus, the safer secondary explosives used in the detonator are much more difficult to detonate but this is used to advantage to assure that random events do not trigger detonation.
The present apparatus further incorporates an exploding wire foil and flyer combination for forming the necessary shock. The exploding wire foil is connected across the electrical circuit which forms the requisite output current. The output current must have a substantial current flow for a minimum interval. It flows through a wire foil which has a narrow neck. In the region of the neck, the current typically vaporizes that portion of the wire foil. When this occurs, the wire foil is exploded. It is arranged so that the foil explosion shears a small disc, called a flyer, which traverses a specified distance to impinge on secondary explosive materials and thereby initiate explosion. This distance is important in providing a safety interlock in the present apparatus. The value of this will be understood on description of the problem set forth below.
The present apparatus is particularly useful in a sealed housing which encloses a set of shaped charges. The sealed container is intended to be leakproof and is constructed in this fashion. It is impossible to know whether it does leak when downhole. When leakage occurs, the leakage will fill the lower part of the closed and sealed housing. When sequential detonation of the shaped charges is started, pressures within the housing rise rapidly. When a noncompressible fluid, partially or wholly, fills the housing, the case will quickly split resulting in destruction of the entire structure and may very well abort the perforating sequence. When this occurs, it may be impossible to retrieve the shattered tool and other equipment on the wireline. It is difficult to know how many of the perforations will be formed. The present detonator is a detonator adapted to be installed at the lower end of the tool. If there is no leakage, there is no fluid in the lower portion of the tool and detonation is triggered through the detonator which sets off the explosive sequence in a detonating cord propagated to the several shaped charges. By contrast, assume that leakage has occurred and that the detonator is then submerged in well fluids. The detonator of the present apparatus is constructed so that well fluids in the tool will prevent electrical firing. First of all, the circuit which provides the necessary current flow to the exploding wire foil has exposed terminals which are fluid shorted to thereby prevent detonation. In addition, the fluid which accumulates in the tool is permitted to come into the detonator to fill the gap between the exploding wire foil and the secondary explosive. This prevents detonation. At the surface, when this occurs, operating personnel will have sufficient information to know that the explosive sequence has not occurred and that the detonator has been prevented from firing. This also enables the entire structure to be retrieved. It is retrieved in an armed, but completely safe, condition since the detonator has been properly prevented from operation by means of the fluid accumulation in the tool.
The present apparatus provides an alternate detonator which has sealed electrical leads. Thus, it can be used fully submerged in well fluids and yet still operate. This particular version of the detonator is desirable when used with perforating shaped charges that are not enclosed in a sealed housing. These are known as "exposed" perforating guns, and any detonator used with them must be fluid tight.
The alternate apparatus similarly contemplates the use of the firing circuit which is an AC Voltage multiplier having a ladder circuit accumulating an increased charge on a charging capacitor. A bleed resistor to ground is included to prevent accumulation of stray or static events. Moreover, the output is through a pair of terminals which are controllably exposed to well fluids. These terminals in turn connect to an exploding wire foil which has the shape of an hourglass so that the narrow portion literally explodes when the current flow is directed through the narrow neck. The exploding wire foil shears a flying disc which is in spaced relationship to a secondary explosive charge. Initiation of the explosive is prompted by impact of the flying disc. The foil end flyer combination is included within a housing which has an internal shoulder abutting the detonating cord so that it is prevented from pumping into the housing by ambient pressure conditions. The housing is sealed at respective spaced ends by means of tapered boots fitting over the exterior. The explosives in the detonator are only secondary explosives thereby providing a significantly safer detonating system.